Using Surface Wave Method to Define a Sinkhole Impact Area in a Noisy Environment

A sinkhole developed at Calvert Cliffs Nuclear Power Plant, Maryland in early 2001. To prevent damage to nearby structures, the sinkhole was quickly filled with dirt (approximately 40 tons). However, the plant had an immediate need to determine if more underground voids existed. The location of the sinkhole was over a groundwater drainage system pipe buried at an elevation of +3 feet (reference is to Chesapeake Bay level). Grade in the sinkhole area is +45 feet. The subsurface drain system is designed to lower the local water table from approximately +20 feet above Bay level to +10 feet. The subsurface drain system is connected to the top of the condenser cooling water discharge conduit at an elevation of -4 feet. The cause of the sinkhole was a subsurface drain pipe that collapsed due to saltwater corrosion of the corrugated metal pipe. The inflow/outflow of sea water and ground water flow caused dirt to be removed from the area where the pipe collapsed. A high-frequency surface-wave survey was conducted to define the sinkhole impact area. Five surface-wave lines were acquired with limited resources: a 24-channel seismograph with a hammer and an aluminum plate as a source. Although the surface-wave survey at Calvert Cliffs Nuclear Power Plant was conducted at a noise level 50-100 times higher than the normal environment for a shallow seismic survey, the shear (S)-wave velocity field calculated from surface-wave data delineated a possible sinkhole impact area. The S-wave velocity field showed chimney-shaped low-velocity anomalies that were directly related to the sinkhole. Based on S-wave velocity field maps, a potential sinkhole impact area was tentatively defined. S-wave velocity field maps also revealed, depending on the acquisition geometry, one side of the water tunnel of the power plant.